The present invention relates to binoculars.
When an object at infinity is observed by a pair of binoculars, a field of view observed by a left eye of an observer and a field of view observed by the right eye substantially overlap each other, and a single field of view is observed when the observer observes the binoculars with both eyes. When an object at a relatively short distance of several meters or less is observed with the binoculars, only a part of the field of view for each of the right eye and left eye overlaps each other, and the observer feels difficulty in observing such an object. This is because, in binoculars, the optical axes of left and right objective lenses are generally fixed to be parallel to each other since the binoculars are generally designed to observe an object located within a range from several tens of meters to infinity. If an object at a short distance is observed with such binoculars, a remarkable discrepancy arises between a focusing condition corresponding to the object (which will be referred to as an adjustment value, i.e., a distance to an object to be focused, for example, represented by a unit of diopter [dptr]=[1/meter]) and convergence value (which is a distance at which a right sight line and a left sight line cross, for example, represented by metric angle [MW]=[1/meter]). When an object is observed at high magnifying power, an influence due to such discrepancy is remarkable. For example, with ten-power binoculars, the discrepancy is ten times in comparison with the discrepancy of naked eyes. The remarkable discrepancy between the adjustment value and convergence value is a burden to the eyes of the observer and causes the eyes to be fatigued. (It should be noted that the term “convergence” means the visual axes of both eyes which are concentrated when observing an object at a short distance, and the angle formed between both axes is referred to as a “convergence angle”).
In view of the above-described problem, in order to reduce the burden to the eyes when observing an object at a short distance, binoculars provided with a convergence value (convergence angle) compensating mechanism have been developed. In such binoculars, in accordance with the adjustment value, the convergence value (or convergence angle) is adjusted by moving both objective lenses in the direction orthogonal to the optical axes thereof to make the objective lenses located close to each other when observing an object at a short distance. Examples of such binoculars are disclosed in Japanese Patent Publications No. 3090007, No. 3196613 and No. 3189328.